Photosynthesis Non-equilibrium redox processes

The cycle is closed by respiration, the process of liberation of chemical energy in the oxidation of organic compoimds  

It is remarkable that in this way a stoichiometric ratio of 1 1 between fixed carbon and released oxygen is established. Therefore, a net oxygen production is only possible when the rate of reaction (2.43) is smaller than that of reaction (2.42), or in other words, the organic matter produced must be buried and protected against oxidation. 

This was the first closed biogeochemical cycle. Before inventing photosynthesis, when fermentation was the only process in transforming organic matter into biomass by primitive life (Eq. 2.36), no cycle was provided. Organic matter, derived either from hydrocarbons found in carbonaceous chondrite material or (as we consider to be less probable) synthesized photochemicals in air, is finally transformed via Equation (2.43) into CO2. 

We assume in the modern world that photosynthesis is balanced with respiration (see also Chapter 2.3.5.3). The reaction (2.42) is of Fischer-Tropsch type synthesis. This is also known from inorganic nature under conditions deep within the earth (Eq. 2.5 and 2.21) and in the upper atmosphere (Eq. 2.35), but under extreme 

Water oxidation in oxygenic photosynthesis is a concerted four-electron process. Thus we can ask was there a transitional multielectron donor before water was adopted as the vmiversal reductant for oxygenic photosynthesis Dismukes et al. (2001) argue that bicarbonate is thermodynamically a better electron donor than water for O2 evolution  

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